1. Field of the Invention
The present invention relates to integrated circuit designs; and, in particular, the present invention relates to integrated circuit designs for image processing.
2. Discussion of the Related Art
The Motion Picture Experts Group (MPEG) is an international committee charged with providing a standard (hereinbelow "MPEG standard") for achieving compatibility between image compression and decompression equipment. This standard specifies both the coded digital representation of video signal for the storage media, and the method for decoding. The representation supports normal speed playback, as well as other playback modes of color motion pictures, and reproduction of still pictures. The MPEG standard covers the common 525- and 625-line television, personal computer and workstation display formats. The MPEG standard is intended for equipment supporting continuous transfer rate of up to 1.5 Mbits per second, such as compact disks, digital audio tapes, or magnetic hard disks. The MPEG standard is intended to support picture frames of approximately 288.times.352 pixels each at a rate between 24 Hz and 30 Hz. A publication by MPEG entitled "Coding for Moving Pictures and Associated Audio for digital storage medium at 1.5 Mbit/s," provides in draft form the proposed MPEG standard, which is hereby incorporated by reference in its entirety to provide detailed information about the MPEG standard.
Under the MPEG standard, the picture is divided into a matrix of "Macroblock slices" (MBS), each MBS containing a number of picture areas (called "macroblocks") each covering an area of 16.times.16 pixels. Each of these picture areas is further represented by one or more 8.times.8 matrices which elements are the spatial luminance and chrominance values. In one representation (4:2:2) of the macroblock, a luminance value (Y type) is provided for every pixel in the 16.times.16-pixel picture area (i.e. in four 8.times.8 "Y" matrices), and chrominance values of the U and V (i.e., blue and red chrominance) types, each covering the same 16.times.16 picture area, are respectively provided in two 8.times.8 "U" and two 8.times.8 "V" matrices. That is, each 8.times.8 U or V matrix has a lower resolution than its luminance counterpart and covers an area of 8.times.16 pixels. In another representation (4:2:0), a luminance value is provided for every pixel in the 16.times.16 pixels picture area, and one 8.times.8 matrix for each of the U and V types is provided to represent the chrominance values of the 16.times.16-pixel picture area. A group of four contiguous pixels in a 2.times.2 configuration is called a "quad pixel"; hence, the macroblock can also be thought of as comprising 64 quad pixels in an 8.times.8 configuration.
The MPEG standard adopts a model of compression and decompression based on lossy compression of both interframe and intraframe information. To compress interframe information, each frame is encoded in one of the following formats: "intra", "predicted", or "interpolated". Intra encoded frames are least frequently provided, the predicted frames are provided more frequently than the intra frames, and all the remaining frames are interpolated frames. In a prediction frame ("P-picture"), only the incremental changes in pixel values from the last I- picture or P-picture are coded. In an interpolation frame ("B- picture"), the pixel values are encoded with respect to both an earlier frame and a later frame. By encoding frames incrementally, using predicted and interpolated frames, the redundancy between frames can be eliminated, resulting in a high efficiency in data storage. Under the MPEG, the motion of an object moving from one screen position to another screen position can be represented by motion vectors. A motion vector provides a shorthand for encoding a spatial translation of a group of pixels, typically a macroblock.
The next steps in compression under the MPEG standard provide lossy compression of intraframe information. In the first step, a 2-dimensional discrete cosine transform (DCT) is performed on each of the 8.times.8 pixel matrices to map the spatial luminance or chrominance values into the frequency domain.
Next, a process called "quantization" weights each element of the 8.times.8 transformed matrix, consisting of 1 "DC" value and sixty-three "AC" values, according to whether the pixel matrix is of the chrominance or the luminance type, and the frequency represented by each element of the transformed matrix. In an I-picture, the quantization weights are intended to reduce to zero many high frequency components to which the human eye is not sensitive. In P- and B- pictures, which contain mostly higher frequency components, the weights are not related to visual perception. Having created many zero elements in the 8.times.8 transformed matrix, each matrix can be represented without further information loss as an ordered list consisting of the "DC" value, and alternating pairs of a non-zero "AC" value and a length of zero elements following the non-zero value. The values on the list are ordered such that the elements of the matrix are presented as if the matrix is read in a zig.sub.-- zag manner (i.e., the elements of a matrix A are read in the order A00, A01, A10, A02, A11, A20 etc.). This representation is space efficient because zero elements are not represented individually.
Finally, an entropy encoding scheme is used to further compress, using variable-length codes, the representations of the DC coefficient and the AC value-run length pairs. Under the entropy encoding scheme, the more frequently occurring symbols are represented by shorter codes. Further efficiency in storage is thereby achieved.
The steps involved in compression under the MPEG standard are computationally intensive. For such a compression scheme to be practical and widely accepted, however, a high speed processor at an economical cost is desired. Such processor is preferably provided in an integrated circuit.
Other standards for image processing exist. These standards include JPEG ("Joint Photographic Expert Group") and CCITT H.261 (also known as "P.times.64"). These standards are available from the respective committees, which are international bodies well-known to those skilled in the art.